South China Sea (SCS) is the largest Western Pacific marginal sea.However,microbial studies have never been performed in the cold seep sediments in the SCS.In 2004,"SONNE" 177 cruise found two cold seep areas with different water depth in the northern SCS.Haiyang 4 area,where the water depth is around 3000 m,has already been confirmed for active seeping on the seafloor,such as microbial mats,authigenic carbonate crusts and bivalves.We investigated microbial abundance and diversity in a 5.55-m sediment core collected from this cold seep area.An integrated approach was employed including geochemistry and 16S rRNA gene phylogenetic analyses.Here,we show that microbial abundance and diversity along with geochemistry profiles of the sediment core revealed a coupled reaction between sulphate reduction and methane oxidatiot.Acridine orange direct count results showed that microbial abundance ranges from 105 to 106 cells/g sediment (wet weight).The depth-related variation of the abundance showed the same trend as the methane concentration profile.Phylogenetic analysis indicated the presence of sulphate-reducing bacteria and anaerobic methane-oxidizing archaea.The diver sity was much higher at the surface,but decreased sharply with depth in response to changes in the geochemical conditions of the sediments,such as methane,sulphate concentration and total organic carbon.Marine Benthic Group B,Chloroflexi and JSI were predominant phylotypes of the archaeal and bacterial libraries,respectively.